Chemically induced sarcomas from nude mice are more immunogenic than similar sarcomas from congenic normal mice

Immunosurveillance shapes the emergence of neo-epitope landscapes of sarcomas, revealing prime targets for immunotherapy

T cells recognize tumor-derived mutated peptides presented on MHC by tumors. The recognition of these neo-epitopes leads to rejection of tumors, an event that is critical for successful cancer immunosurveillance. Determination of tumor-rejecting neo-epitopes in human tumors has proved difficult, though recently developed systems approaches are becoming increasingly useful at evaluating their immunogenicity. We have used the differential aggretope index to determine the neo-epitope burden of sarcomas and observed a conspicuously titrated antigenic landscape, ranging from the highly antigenic osteosarcomas to the low antigenic leiomyosarcomas and liposarcomas. We showed that the antigenic landscape of the tumors inversely reflected the historical T cell responses in the tumor-bearing patients. We predicted that highly antigenic tumors with poor antitumor T cell responses, such as osteosarcomas, would be responsive to T cell-based immunotherapy regimens and demonstrated this in a murine osteosarcoma model. Our study presents a potentially novel pipeline for determining antigenicity of human tumors, provides an accurate predictor of potential neo-epitopes, and will be an important indicator of which cancers to target with T cell-enhancing immunotherapy.

Recent Advances of RNA m6A Modifications in Cancer Immunoediting and Immunotherapy

Cancer immunotherapy, which modulates immune responses against tumors using immune-checkpoint inhibitors or adoptive cell transfer, has emerged as a novel and promising therapy for tumors. However, only a minority of patients demonstrate durable responses, while the majority of patients are resistant to immunotherapy. The immune system can paradoxically constrain and promote tumor development and progression. This process is referred to as cancer immunoediting. The mechanisms of resistance to immunotherapy seem to be that cancer cells undergo immunoediting to evade recognition and elimination by the immune system. RNA modifications, specifically N6-methyladenosine (m6A) methylation, have emerged as a key regulator of various post-transcriptional gene regulatory processes, such as RNA export, splicing, stability, and degradation, which play unappreciated roles in various physiological and pathological processes, including immune system development and cancer pathogenesis. Therefore, a deeper understanding of the mechanisms by which RNA modifications impact the cancer immunoediting process can provide insight into the mechanisms of resistance to immunotherapies and the strategies that can be used to overcome such resistance. In this chapter, we briefly introduce the background of cancer immunoediting and immunotherapy. We also review and discuss the roles and mechanisms of RNA m6A modifications in fine-tuning the innate and adaptive immune responses, as well as in regulating tumor escape from immunosurveillance. Finally, we summarize the current strategies targeting m6A regulators for cancer immunotherapy.

Agents of cancer immunosurveillance: HSPs and dsDNA

Tumor immunosurveillance requires tumor cell-derived molecules to initiate responses through corresponding receptors on antigen presenting cells (APCs) and a specific effector response designed to eliminate the emerging tumor cells. This is supported by evidence from immunodeficient individuals and experimental animals. Recent discoveries suggest that adjuvanticity of tumor-derived heat shock proteins (HSPs) and double-stranded DNA (dsDNA) are necessary for tumor-specific immunity. There is also the obligatory early transfer of tumor antigens to APCs. We argue that tumor-derived HSPs deliver sufficient chaperoned antigen for cross-priming within the quantitative limits set by nascent tumors. In contrast to late-stage tumors, we are only just beginning to understand the unique interactions of the immune system with precancerous/nascent neoplastic cells, which is important for improved cancer prevention measures.

Immunotherapeutic Strategies in Cancer and Atherosclerosis-Two Sides of the Same Coin

The development and clinical approval of immunotherapies has revolutionized cancer therapy. Although the role of adaptive immunity in atherogenesis is now well-established and several immunomodulatory strategies have proven beneficial in preclinical studies, anti-atherosclerotic immunotherapies available for clinical application are not available. Considering that adaptive immune responses are critically involved in both carcinogenesis and atherogenesis, immunotherapeutic approaches for the treatment of cancer and atherosclerosis may exert undesirable but also desirable side effects on the other condition, respectively. For example, the high antineoplastic efficacy of immune checkpoint inhibitors, which enhance effector immune responses against tumor cells by blocking co-inhibitory molecules, was recently shown to be constrained by substantial proatherogenic properties. In this review, we outline the specific role of immune responses in the development of cancer and atherosclerosis. Furthermore, we delineate how current cancer immunotherapies affect atherogenesis and discuss whether anti-atherosclerotic immunotherapies may similarly have an impact on carcinogenesis.

The PARP Enzyme Family and the Hallmarks of Cancer Part 2: Hallmarks Related to Cancer Host Interactions

Poly (ADP-ribose) polymerases (PARPs) modify target proteins with a single ADP-ribose unit or with a poly (ADP-ribose) (PAR) polymer. PARP inhibitors (PARPis) recently became clinically available for the treatment of BRCA1/2 deficient tumors via the synthetic lethality paradigm. This personalized treatment primarily targets DNA damage-responsive PARPs (PARP1-3). However, the biological roles of PARP family member enzymes are broad; therefore, the effects of PARPis should be viewed in a much wider context, which includes complex effects on all known hallmarks of cancer. In the companion paper (part 1) to this review, we presented the fundamental roles of PARPs in intrinsic cancer cell hallmarks, such as uncontrolled proliferation, evasion of growth suppressors, cell death resistance, genome instability, replicative immortality, and reprogrammed metabolism. In the second part of this review, we present evidence linking PARPs to cancer-associated inflammation, anti-cancer immune response, invasion, and metastasis. A comprehensive overview of the roles of PARPs can facilitate the identification of novel cancer treatment opportunities and barriers limiting the efficacy of PARPi compounds.

Impact of TCR Diversity on the Development of Transplanted or Chemically Induced Tumors

Burnet postulated that the diversity of T-cell receptors (TCR) allows T cells to protect against the development of cancers that display antigens with a similar, seemingly endless diversity. To test this hypothesis, we developed a strategy in which a single breeding pair of mice gives rise to four groups of sibling mice. Three of the four groups had a similar number of CD8⁺ T cells, but TCR diversity was either broad, significantly reduced, or absent when expressing only one type of TCR. The fourth group had no T cells. All mice shared the same housing, and, therefore, their microbial environment was similar. Only slight differences in the intestinal flora were observed under these conditions. An undisturbed broad TCR repertoire was required for the rejection of inoculated cancers displaying the natural antigenic heterogeneity of primary tumors, whereas even one type of TCR was sufficient to protect against artificial cancers stably expressing cognate antigens. The three groups of mice with limited or no TCR repertoire showed an increased risk of developing primary tumors after chemical induction. However, the risk of early death or morbidity in these cohorts of mice was significantly higher than in mice with a diverse TCR repertoire, and it remains unknown whether mice with reduced TCR diversity, who died early without cancer, would have developed tumors with higher, lower, or equal probability after induction. Together, TCR diversity seems crucial to overcome the natural genetic instability of cancers and their antigenic heterogeneity, which impacts the design of cellular therapies.

Frequent Loss of IRF2 in Cancers Leads to Immune Evasion through Decreased MHC Class I Antigen Presentation and Increased PD-L1 Expression

To arise and progress, cancers need to evade immune elimination. Consequently, progressing tumors are often MHC class I (MHC-I) low and express immune inhibitory molecules, such as PD-L1, which allows them to avoid the main antitumor host defense, CD8⁺ T cells. The molecular mechanisms that led to these alterations were incompletely understood. In this study, we identify loss of the transcription factor IRF2 as a frequent underlying mechanism that leads to a tumor immune evasion phenotype in both humans and mice. We identified IRF2 in a CRISPR-based forward genetic screen for genes that controlled MHC-I Ag presentation in HeLa cells. We then found that many primary human cancers, including lung, colon, breast, prostate, and others, frequently downregulated IRF2. Although IRF2 is generally known as a transcriptional repressor, we found that it was a transcriptional activator of many key components of the MHC-I pathway, including immunoproteasomes, TAP, and ERAP1, whose transcriptional control was previously poorly understood. Upon loss of IRF2, cytosol-to-endoplasmic reticulum peptide transport and N-terminal peptide trimming become rate limiting for Ag presentation. In addition, we found that IRF2 is a repressor of PD-L1. Thus, by downregulating a single nonessential gene, tumors become harder to see (reduced Ag presentation), more inhibitory (increased checkpoint inhibitor), and less susceptible to being killed by CD8⁺ T cells. Importantly, we found that the loss of Ag presentation caused by IRF2 downregulation could be reversed by IFN-stimulated induction of the transcription factor IRF1. The implication of these findings for tumor progression and immunotherapy are discussed.

Have Cells Harboring the HIV Reservoir Been Immunoedited?

Immunoediting is an important concept in oncology, delineating the mechanisms through which tumors are selected for resistance to immune-mediated elimination. The recent emergence of immunotherapies, such as checkpoint inhibitors, as pillars of cancer therapy has intensified interest in immunoediting as a constraint limiting the efficacy of these approaches. Immunoediting manifests at a number of levels for different cancers, for example through the establishment of immunosuppressive microenvironments within solid tumors. Of particular interest to the current review, selection also occurs at the cellular level; and recent studies have revealed novel mechanisms by which tumor cells acquire intrinsic resistance to immune recognition and elimination. While the selection of escape mutations in viral epitopes by HIV-specific T cells, which is a hallmark of chronic HIV infection, can be considered a form of immunoediting, few studies have considered the possibility that HIV-infected cells themselves may parallel tumors in having differential intrinsic susceptibilities to immune-mediated elimination. Such selection, on the level of an infected cell, may not play a significant role in untreated HIV, where infection is propagated by high levels of cell-free virus produced by cells that quickly succumb to viral cytopathicity. However, it may play an unappreciated role in individuals treated with effective antiretroviral therapy where viral replication is abrogated. In this context, an "HIV reservoir" persists, comprising long-lived infected cells which undergo extensive and dynamic clonal expansion. The ability of these cells to persist in infected individuals has generally been attributed to viral latency, thought to render them invisible to immune recognition, and/or to their compartmentalization in anatomical sites that are poorly accessible to immune effectors. Recent data from ex vivo studies have led us to propose that reservoir-harboring cells may additionally have been selected for intrinsic resistance to CD8+ T cells, limiting their elimination even in the context of antigen expression. Here, we draw on knowledge from tumor immunoediting to discuss potential mechanisms by which clones of HIV reservoir-harboring cells may resist elimination by CD8+ T cells. The establishment of such parallels may provide a premise for testing therapeutics designed to sensitize tumor cells to immune-mediated elimination as novel approaches aimed at curing HIV infection.

CD91 on dendritic cells governs immunosurveillance of nascent, emerging tumors

The immune system detects aberrant, premalignant cells and eliminates them before the development of cancer. Immune cells, including T cells, have been shown to be critical components in eradicating these aberrant cells, and when absent in the host, incidence of cancer increases. Here, we show that CD91, a receptor expressed on antigen-presenting cells, is required for priming immune responses to nascent, emerging tumors. In the absence of CD91, effector immune responses are subdued, and tumor incidence and progression are amplified. We also show that, consequently, tumors that arise in the absence of CD91 express neo-epitopes with indices that are indicative of greater immunogenicity. Polymorphisms in human CD91 that are expected to affect ligand binding are shown to influence antitumor immune responses in cancer patients. This study presents a molecular mechanism for priming immune responses to nascent, emerging tumors that becomes a predictor of cancer susceptibility and progression.

Progression from low- to high-grade in a glioblastoma model reveals the pivotal role of immunoediting

The mutual reshape of tumor and immune system cells during tumor progression is a widely accepted notion in different cancers including gliomas. The importance of this phenomenon in shaping glioma progression and the mechanisms governing it, however, are not fully elucidated. Taking advantage of a well-characterized in vivo glioma model we performed an analysis of glioma cells transcriptomes at different stages of progression and unveiled the reorganization of glioma-immune system interactions. Specifically, we show that the inability of low-grade glioma cells to orthotopically graft in syngeneic immunocompetent mice, positively correlates with the abundance of infiltrating lymphocytes in donor tumors and with a highly immunostimulatory transcriptional profile. Notably, during tumor progression glioma cells downregulate these genes and the immune infiltrate shifts towards a pro-tumorigenic phenotype. Challenging low-grade gliomas by grafting into immunodeficient hosts revealed the crucial role of the adaptive immune system in constraining glioma progression. Finally, we observed that although progression still takes place in immunodeficient mice, it is slower, likely due to a milder selection thus reinforcing the view of a pivotal role for the immune system in regulating glioma progression.

The possibility of cancer immune editing in gliomas. A critical review

The relationship between anti-tumoral immunity and cancer progression is complex. Recently, immune editing has emerged as a model to explain the interplay between the immune system and the selection of genetic alterations in cancer. In this model, the immune system selects cancer cells that grow as these are fit to escape immune surveillance during tumor development. Gliomas and glioblastoma, the most aggressive and most common of all primary malignant brain tumors are genetically heterogeneous, are relatively less antigenic, and are less responsive to immunotherapy than other cancers. In this review, we provide an overview of the relationship between glioma´s immune suppressive features, anti-tumoral immunity and cancer genomics. In this context, we provide a critical discussion of evidence suggestive of immune editing in this disease and discuss possible alternative explanations for these findings.

Immune Checkpoint Blockade in Breast Cancer Therapy

Cancer immunotherapy is emerging as the most promising novel strategy for cancer treatment. Cancer immunotherapy is broadly categorized into three forms: immune checkpoint modulation, adoptive cell transfer, and cancer vaccine. Immune checkpoint blockade is demonstrated as the most clinically effective treatment with low immune-related adverse events (irAE). Blockade of PD-1/PD-L1 and CTLA-4 has achieved remarkable success in treating various types of tumors, which sparks great interests in this therapeutic strategy and expands the role of immune checkpoint blockade in treating tumors including breast cancer. Based on the notable results obtained from clinical trials, the United States' Food and Drug Administration (FDA) has approved multiple CTLA-4 monoclonal antibodies as well as the PD-1/PD-L1 monoclonal antibodies for treatment of different types of tumors. The theories of immunoediting, T-cell exhaustions, and co-stimulatory/co-inhibitory pathways are immunological foundations for immune checkpoint blockade therapy. Breast cancers such as triple negative breast cancer and HER-2 negative breast cancer respond to immune checkpoint blockade therapy due to their high immunogenicity. PD-1/PD-L1 blockade has just received FDA approval as a standard cancer therapy for solid tumors such as breast cancer. Development of immune checkpoint blockade focuses on two directions: one is to identify proper biomarkers of immune checkpoint blockade in breast cancer, and the other is to combine therapies with PD-1/PD-L1 blockade antibodies to achieve optimal clinical outcomes.

Cancer risk in immune-mediated inflammatory diseases (IMID)

Inflammation and cancer have a profound yet ambiguous relationship. Inflammation - especially chronic inflammation - has protumorigenic effects, but inflammatory cells also mediate an immune response against the tumor and immunosuppression is known to increase the risk for certain tumors.This article reviews current literature on the role of inflammation in cancer and the cancer risk in immune-mediated inflammatory diseases (IMIDs). We discuss the effect on cancer risk of different drug classes used in the treatment of IMIDs treatment, including biologicals such as tumor necrosis factor (TNF) inhibitors.Overall cancer incidence and mortality risk are similar to the general population in inflammatory bowel disease (IBD), and slightly increased for rheumatoid arthritis and psoriasis, with risk profiles differing for different tumor types. Increased risk for non-melanoma skin cancer is associated with thiopurine treatment in IBD, with the combination of anti-TNF and methotrexate in rheumatoid arthritis and with PUVA, cyclosporine and anti-TNF treatment in psoriasis. Data on the safety of using biologic or immunosuppressant therapy in IMID patients with a history of cancer are scarce.This review provides clinicians with a solid background to help them in making decisions about treatment of immune-mediated diseases in patients with a tumor history.This article is related to another review article in Molecular Cancer: http://www.molecular-cancer.com/content/12/1/86.

Can exercise-related improvements in immunity influence cancer prevention and prognosis in the elderly?

Cancer incidence increases with advancing age. Over 60% of new cancers and 70% of cancer deaths occur in individuals aged 65 years or older. One factor that may contribute to this is immunosenescence - a canopy term that is used to describe age-related declines in the normal functioning of the immune system. There are multiple age-related deficits in both the innate and adaptive systems that may play a role in the increased incidence of cancer. These include decreased NK-cell function, impaired antigen uptake and presentation by monocytes and dendritic cells, an increase in 'inflammaging', a decline in the number of naïve T-cells able to respond to evolving tumor cells, and an increase in functionally exhausted senescent cells. There is consensus that habitual physical exercise can offer protection against certain types of cancer; however the evidence linking immunological mechanisms, exercise, and reduced cancer risk remain tentative. Multiple studies published over the last two decades suggest that exercise can mitigate the deleterious effects of age on immune function, thus increasing anti-cancer immunity. The potential ameliorative effect of exercise on these mechanisms include evidence that physical activity is able to stimulate greater NK-cell activity, enhance antigen-presentation, reduce inflammation, and prevent senescent cell accumulation in the elderly. Here we discuss the role played by the immune system in preventing and controlling cancer and how aging may retard these anti-cancer mechanisms. We also propose a pathway by which exercise-induced alterations in immunosenescence may decrease the incidence of cancer and help improve prognosis in cancer patients.

From tumor cell metabolism to tumor immune escape

Tumorigenesis implies adaptation of tumor cells to an adverse environment. First, developing tumors must acquire nutrients to ensure their rapid growth. Second, they must escape the attack from the host immune system. Recent studies suggest that these phenomena could be related and that tumor cell metabolism may propel tumor immune escape. Tumor cell metabolism tends to avoid mitochondrial activity and oxidative phosphorylation (OXPHOS), and largely relies on glycolysis to produce energy. This specific metabolism helps tumor cells to avoid the immune attack from the host by blocking or avoiding the immune attack. By changing their metabolism, tumor cells produce or sequester a variety of amino acids, lipids and chemical compounds that directly alter immune function therefore promoting immune evasion. A second group of metabolism-related modification targets the major histocompatibility complex-I (MHC-I) and related molecules. Tumor MHC-I presents tumor-associated antigens (TAAs) to cytotoxic T-cells (CTLs) and hence, sensitizes cancer cells to the cytolytic actions of the anti-tumor adaptive immune response. Blocking tumor mitochondrial activity decreases expression of MHC-I molecules at the tumor cell surface. And peroxynitrite (PNT), produced by tumor-infiltrating myeloid cells, chemically modifies MHC-I avoiding TAA expression in the plasma membrane. These evidences on the role of tumor cell metabolism on tumor immune escape open the possibility of combining drugs designed to control tumor cell metabolism with new procedures of anti-tumor immunotherapy. This article is part of a Directed Issue entitled: Bioenergetic dysfunction, adaptation and therapy.

Targeting stroma to treat cancers

All cancers depend on stroma for support of growth. Leukemias, solid tumors, cancer cells causing effusions, metastases as well as micro-disseminated cancer cells release factors that stimulate stromal cells, which in turn produce ligands that stimulate cancer cells. Therefore, elimination of stromal support by destroying the stromal cells or by inhibiting feedback stimulation of cancer growth is in the focus of many evolving therapies. A stringent evaluation of the efficacy of stromal targeting requires testing in animal models. Most current studies emphasize the successes of stromal targeting rather than deciphering its limitations. Here we show that many of the stromal targeting approaches, while often reducing tumor growth rates, are rarely curative. Therefore, we will also discuss conditions where stromal targeting can eradicate large established tumors. Finally, we will examine still unanswered questions of this promising and exciting area of cancer research.

Inflammation and immune surveillance in cancer

Chronic inflammation is a risk factor for tumor development. However, understanding the effect of the immune system on tumor development has only been significantly advanced over the past two decades. We now appreciate that the immune system, in addition to tumor-suppressive function by eliminating nascent transformed tumor cells, can also exert selection pressure on tumor cells and facilitate tumor growth by providing a favorable tumor microenvironment. Yet, the distinctions between tumor-promoting inflammation and tumor-suppressive immunity are still not clear due to the dual role of some cytokines and other molecules in the immune system. The danger signal hypothesis has shaped our view of the role of immunity in cancer development, but still little is known about the exact role of danger signal receptors in cancer progression. In this review, we introduce the processes of cancer immunoediting and inflammation-induced cancer and discuss what is currently known about the role of danger signal receptors in cancer development and progression.

T cell surveillance of oncogene-induced prostate cancer is impeded by T cell-derived TGF-β1 cytokine

Tolerance induction in T cells takes place in most tumors and is thought to account for tumor evasion from immune eradication. Production of the cytokine TGF-β is implicated in immunosuppression, but the cellular mechanism by which TGF-β induces T cell dysfunction remains unclear. With a transgenic model of prostate cancer, we showed that tumor development was not suppressed by the adaptive immune system, which was associated with heightened TGF-β signaling in T cells from the tumor-draining lymph nodes. Blockade of TGF-β signaling in T cells enhanced tumor antigen-specific T cell responses and inhibited tumor development. Surprisingly, T cell- but not Treg cell-specific ablation of TGF-β1 was sufficient to augment T cell cytotoxic activity and blocked tumor growth and metastases. These findings reveal that T cell production of TGF-β1 is an essential requirement for tumors to evade immunosurveillance independent of TGF-β produced by tumors.

Downregulation of IFN-gammaR in association with loss of Fas function is linked to tumor progression

The host immune system functions as an intrinsic surveillance network in the recognition and destruction of tumor cells, and it has been demonstrated that lymphocytes and IFN-gamma are the primary tumor suppressors of the immune system. However, the immune system can concurrently select for tumor variants with reduced immunogenicity and aggressive phenotypes. We report here that tumor escape variants that have survived CTL adoptive immunotherapy exhibited decreased expression levels of both Fas and IFN-gammaR in vitro. Furthermore, examination of spontaneously arising mouse primary mammary carcinoma and lung metastases revealed that both Fas and IFN-gammaR protein levels were dramatically lower in lung metastases than in primary tumors in vivo. Functional disruption of either the Fas- or the IFN-gamma signaling pathway enhanced the colonization efficiency of preexisting metastatic tumor cells, whereas disruption of both Fas and IFN-gammaR pathways resulted in synergistic augmentation of the colonization efficiency of the preexisting metastatic tumor cells, as determined by experimental lung metastases assay. Gene expression profiling revealed that altered expression of genes involved in immediate IFN-gammaR signaling, the interferon primary response, apoptosis and tumor colonization is associated with loss of IFN-gammaR function and enhanced metastatic potential. Interestingly, disruption of IFN-gammaR function did not alter tumor cell susceptibility to CTL-mediated cytotoxicity, but is linked to enhanced infiltration of endogenous T cells in the tumor microenvironment in vivo. These findings suggest that coordinate downregulation of Fas and IFN-gammaR, 2 key components of cancer immunosurveillance system on tumor cells, leads to a more aggressive metastatic phenotype.

MHC class II-transduced tumor cells originating in the immune-privileged eye prime and boost CD4(+) T lymphocytes that cross-react with primary and metastatic uveal melanoma cells

Uveal melanoma, the most common malignancy of the eye, has a 50% rate of liver metastases among patients with large primary tumors. Several therapies prolong survival of metastatic patients; however, none are curative and no patients survive. Therefore, we are exploring immunotherapy as an alternative or adjunctive treatment. Uveal melanoma may be particularly appropriate for immunotherapy because primary tumors arise in an immune-privileged site and may express antigens to which the host is not tolerized. We are developing MHC class II (MHC II)-matched allogeneic, cell-based uveal melanoma vaccines that activate CD4(+) T lymphocytes, which are key cells for optimizing CD8(+) T-cell immunity, facilitating immune memory, and preventing tolerance. Our previous studies showed that tumor cells genetically modified to express costimulatory and MHC II molecules syngeneic to the recipient are potent inducers of antitumor immunity. Because the MHC II-matched allogeneic vaccines do not express the accessory molecule, Invariant chain, they present MHC II-restricted peptides derived from endogenously encoded tumor antigens. We now report that MHC II-matched allogeneic vaccines, prepared from primary uveal melanomas that arise in the immune-privileged eye, prime and boost IFNgamma-secreting CD4(+) T cells from the peripheral blood of either healthy donors or uveal melanoma patients that cross-react with primary uveal melanomas from other patients and metastatic tumors. In contrast, vaccines prepared from metastatic cells in the liver are less effective at activating CD4(+) T cells, suggesting that tumor cells originating in immune-privileged sites may have enhanced capacity for inducing antitumor immunity and for serving as immunotherapeutic agents.

Immune surveillance of tumors

The ability of the immune system to identify and destroy nascent tumors, and to thereby function as a primary defense against cancer, has been debated for many decades. Recent findings by a number of investigators in both mouse models of cancer and humans with cancer now offer compelling evidence that particular immune cell types, effector molecules, and pathways can sometimes collectively function as extrinsic tumor suppressor mechanisms. This work provides the basis for further study of natural immunity to cancer and for rational use of this information in the design of immunotherapies in combination with other conventional cancer treatments.

HER-2/neu antigen loss and relapse of mammary carcinoma are actively induced by T cell-mediated anti-tumor immune responses

Induction of tumor-specific immune responses results in the inhibition of tumor development. However, tumors recur because of the tumor immunoediting process that facilitates development of escape mechanisms in tumors. It is not known whether tumor escape is an active process whereby anti-tumor immune responses induce loss or downregulation of the target antigen in the antigen-positive clones. To address this question, we used rat neu-overexpressing mouse mammary carcinoma (MMC) and its relapsed neu antigen-negative variant (ANV). ANV emerged from MMC under pressure from neu-specific T cell responses in vivo. We then cloned residual neu antigen-negative cells from MMC and residual neu antigen-positive cells from ANV. We found marked differences between these neu-negative clones and ANV, demonstrating that the residual neu-negative clones are probably not the origin of ANV. Since initial rejection of MMC was associated with the presence of IFN-gamma-secreting T cells, we treated MMC with IFN-gamma and showed that IFN-gamma could induce downregulation of neu expression in MMC. This appears to be due to methylation of the neu promoter. Together, these data suggest that neu antigen loss is an active process that occurs in primary tumors due to the neu-targeted anti-tumor immune responses.

Cancer immunoediting from immune surveillance to immune escape

Cancer immune surveillance is considered to be an important host protection process to inhibit carcinogenesis and to maintain cellular homeostasis. In the interaction of host and tumour cells, three essential phases have been proposed: elimination, equilibrium and escape, which are designated the 'three E's'. Several immune effector cells and secreted cytokines play a critical role in pursuing each process. Nascent transformed cells can initially be eliminated by an innate immune response such as by natural killer cells. During tumour progression, even though an adaptive immune response can be provoked by antigen-specific T cells, immune selection produces tumour cell variants that lose major histocompatibility complex class I and II antigens and decreases amounts of tumour antigens in the equilibrium phase. Furthermore, tumour-derived soluble factors facilitate the escape from immune attack, allowing progression and metastasis. In this review, the central roles of effector cells and cytokines in tumour immunity, and the escape mechanisms of tumour cells during tumour progression are discussed.

Cancer immunoediting from immune surveillance to immune escape

Cancer immune surveillance is considered to be an important host protection process to inhibit carcinogenesis and to maintain cellular homeostasis. In the interaction of host and tumour cells, three essential phases have been proposed: elimination, equilibrium and escape, which are designated the 'three E's'. Several immune effector cells and secreted cytokines play a critical role in pursuing each process. Nascent transformed cells can initially be eliminated by an innate immune response such as by natural killer cells. During tumour progression, even though an adaptive immune response can be provoked by antigen-specific T cells, immune selection produces tumour cell variants that lose major histocompatibility complex class I and II antigens and decreases amounts of tumour antigens in the equilibrium phase. Furthermore, tumour-derived soluble factors facilitate the escape from immune attack, allowing progression and metastasis. In this review, the central roles of effector cells and cytokines in tumour immunity, and the escape mechanisms of tumour cells during tumour progression are discussed.

Intrathymic selection: new insight into tumor immunology

Central tolerance to self-antigens is formed in the thymus where deletion of clones with high affinity to "self" takes place. Expression of peripheral antigens in the thymus has been implicated in T cell tolerance and autoimmunity. During the last years, it has been shown that medullary thymic epithelial cells (mTECs) are the unique cell type expressing a diverse range of tissue-specific antigens. Promiscuous gene expression is a cell autonomous property of thymic epithelial cells and is maintained during the entire period of thymic T cell output. The array of promiscuously expressed self-antigens was random and included well-known targets for cancer immunotherapy, such as alpha-fetoprotein, P1A, tyrosinase, and gp100. Gene expression in normal tissues may result in tolerance of high-avidity cytotoxic T lymphocyte (CTL), leaving behind low-avidity CTL that cannot provide effective immunity against tumors expressing the relevant target antigens. Thus, it may be evident that tumor vaccines that targeted the tumor-associated antigens should be inefficient due to the loss of high-avidity T cell clones capable to be stimulated. Stauss with colleagues have described a strategy to circumvent immunological tolerance that can be used to generate high-avidity CTL against self-proteins, including human tumor-associated antigens. In this strategy, the allorestricted repertoire of T cells from allogenic donor is used as a source of T cell clones with high avidity to tumor antigens of recipient for adoptive immunotherapy. Then, the T cell receptor (TCR) genes isolated from antigen-specific T cells can be exploited as generic therapeutic molecules for antigen-specific immunotherapy.

Immunoepigenetics: the unseen side of cancer immunoediting

Cancer immunosurveillance representing, till recently, the explanatory framework relating cancer and the immune system, does not convincingly explain tumor escape. At the beginning of the decade, a new theory emerged, namely the immunoediting theory, and it comprehensively defines the role of the immune system in carcinogenesis. The core of this theory embraces the concept that the immune system on the one hand protects the body from cancer and on the other it shapes the immunogenicity of these cancers, thus presents a persuasive rationalization of the resistance of tumors against the immune response. With the immune system playing, in this context, such a pivotal role in shaping the tumor immune profile and in subsequent oncogenesis, it seems rather paradoxical to accept the immunocompetent host's immune system as a constant moiety. While DNA mutations of immune genes create a rather polymorphic condition, their frequency is much lower than that of other genetic events. Of these, epigenetic alterations give rise to new epialleles, which can reach up to 100% per locus. Bearing in mind that cancer is characterized by a tremendous amount of epigenetic aberrations, in both gene and global level, it is reasonable to postulate that, for the same unknown causes, analogous aberrations could affect the immune genes. Should this be the case, the relation between oncogenesis and the immune system appears much more dynamic and complex. Such an immunoepigenetic approach to carcinogenesis could improve our understanding of a series of common cancer-related aspects, such as environmental risk factors, effectiveness of demethylating agents, failure of current immunotherapies, etc. Moreover, this immunoepigenetic paradigm will take the current perception of the immune system and cancer interrelation further and beyond, constituting that the immunoresistant cancer cell phenotype is not shaped by the immune system acting as a steady and rigid evolutionary pressure, but rather as an extremely dynamic variable.

Innate Immune Recognition and Suppression of Tumors

In this chapter, we first summarized the strong evidence that now supports the existence of an effective cancer immune surveillance process that prevents cancer development in both mice and humans. We then focused the remainder of the chapter on methods of tumor recognition that contribute to natural host immune suppression of tumors. In particular, NKG2D is a type II transmembrane-anchored glycoprotein expressed as a disulfide-linked homodimer on the surface of all mouse and human natural killer cells (NK cells). Stimulation of NK cell through NKG2D triggers cell-mediated cytotoxicity and in some cases induces production of cytokines. NKG2D binds to family of ligands with structural homology to major histocompatibility complex (MHC) class I, however, NKG2D ligands often display upregulated surface expression on stressed cells and are frequently overexpressed by tumors unlike conventional MHC class I molecules. Evidence clearly implicate that NKG2D recognition plays an important role in tumor immune surveillance.

Cancer Immunosurveillance and Immunoediting: The Roles of Immunity in Suppressing Tumor Development and Shaping Tumor Immunogenicity

Cellular transformation and tumor development result from an accumulation of mutational and epigenetic changes that alter normal cell growth and survival pathways. For the last 100 years, there has been a vigorous debate as to whether the unmanipulated immune system can detect and eliminate such altered host derived cells despite the fact that cancer cells frequently express either abnormal proteins or abnormal levels of normal cellular proteins that function as tumor antigens. In this review, we discuss the current state of this argument and point out some of the recent key experiments demonstrating that immunity not only protects the host from cancer development (i.e., provides a cancer immunosurveillance function) but also can promote tumor growth, sometimes by generating more aggressive tumors. The terminology "cancer immunoediting" has been used to describe this dual host protective and tumor promoting action of immunity, and herein we summarize the ever-increasing experimental and clinical data that support the validity of this concept.

Tumor-host immune interactions and dendritic cell dysfunction

Several lines of evidence from recent years support the existence of cancer immunosurveillance, especially studies of natural killer (NK) cells and the IFN-gamma pathway. However, immune suppression is clearly observed in cancer patients and tumor-bearing animals as well. The fact is that although cancers often elicit a vigorous immune response during the early part of their growth, the immune response is soon down-regulated, permitting progressive tumor growth. Apparently, the intrinsic plasticity of tumors allows the immune system to sculpt the immunogenic phenotypes of tumors to escape efficient immune destruction. But most evidently, several mechanisms have now been found to contribute to the failure of immune control of tumor growth. Tumor cells have a very low level of MHC class II, costimulatory molecules, and weak antigens. They also produce immune suppressive factors (VEGF, IL-10, PGE(2)) that exert systemic effects on immune cell function. In particular, disabled dendritic cell differentiation, maturation, migration, and function are fundamental to this defect, as they are the most potent antigen-presenting cells (APCs) of the immune system, interacting with T and B lymphocyte as well as NK cells to induce and modulate immune responses. In addition, tumors also alter host hematopoiesis and produce large numbers of immature dendritic cells, and evidence shows that these cells are directly immune suppressive. Harnessing the immune system for effective cancer therapy has remained a great challenge. DC-based vaccines, or DC-based vaccines in combination with treatments designed to improve the host immune environment, may offer hope for more effective cancer immunotherapy. Tumor-host interactions are an important determinant of tumor behavior and response to therapy. How tumors interact with their hosts is thus a very broad and complex topic. In this chapter, we will focus on tumor-host immune interactions and the roles of dendritic cell dysfunction in tumor avoidance of host immune responses. We will survey recent findings regarding tumor immune surveillance, antitumor host immune responses, and how the immune system also functions to promote or select tumor variants with reduced immunogenicity. We will then discuss immune suppression caused by tumors, which is clearly observed in tumor-bearing animals and cancer patients. Finally, we will discuss altered dendritic cell function and differentiation in some detail, as it is likely to be one of the most fundamental mechanisms by which tumors escape immune responses.

The immunobiology of cancer immunosurveillance and immunoediting

The last fifteen years have seen a reemergence of interest in cancer immunosurveillance and a broadening of this concept into one termed cancer immunoediting. The latter, supported by strong experimental data derived from murine tumor models and provocative correlative data obtained by studying human cancer, holds that the immune system not only protects the host against development of primary nonviral cancers but also sculpts tumor immunogenicity. Cancer immunoediting is a process consisting of three phases: elimination (i.e., cancer immunosurveillance), equilibrium, and escape. Herein, we summarize the data supporting the existence of each of the three cancer immunoediting phases. The full understanding of the immunobiology of cancer immunosurveillance and immunoediting will hopefully stimulate development of more effective immunotherapeutic approaches to control and/or eliminate human cancers.

The critical role of type-1 innate and acquired immunity in tumor immunotherapy

The discovery of a large array of tumor antigens has demonstrated that host lymphocytes can indeed recognize and destroy tumor cells as originally proposed in the cancer immunosurveillance hypothesis. Recent reports that led to the cancer immunoediting concept also strongly support the immunosurveillance hypothesis, and they further indicate that the host immune system plays a critical role not only in promoting host protection against cancer but also in selecting tumors that can better escape from immune attack. Thus, it is now clear that T cells have the ability to recognize and destroy spontaneously arising tumors. However, the generation of antitumor immunity is often difficult in the tumor-bearing host because of various negative regulatory mechanisms. Here, we review our recent work on tumor immunotherapy, which utilizes the activation of type-1 innate and/or acquired immunity as a potent strategy to overcome immunosup-pression in the tumor-bearing host. We have established a variety of tumor therapeutic protocols that aim to activate type-1 immunity, such as tumor-vaccine therapy with CpG encapsulat-ed in liposomes, cell therapy using tumor-specific Th1 cells, and gene therapy using gene-engineered Th1 cells. We found that CpG encapsulated in liposomes stimulated IL-12-producing DC and induced IFN-gamma-producing NK cells, NKT cells, and tumor-specific CTL. Th1 cell therapy was also shown to be beneficial for acceleration of APC/Th1 cell-cell interaction in the DLN, which was critical for inducing tumor-specific CTL at the tumor site. Therefore, we conclude that the activation of type-1 innate and acquired immunity is crucial for tumor immunotherapy in order to overcome strong immunosuppression in the tumor-bearing host.

The three Es of cancer immunoediting

After a century of controversy, the notion that the immune system regulates cancer development is experiencing a new resurgence. An overwhelming amount of data from animal models--together with compelling data from human patients--indicate that a functional cancer immunosurveillance process indeed exists that acts as an extrinsic tumor suppressor. However, it has also become clear that the immune system can facilitate tumor progression, at least in part, by sculpting the immunogenic phenotype of tumors as they develop. The recognition that immunity plays a dual role in the complex interactions between tumors and the host prompted a refinement of the cancer immunosurveillance hypothesis into one termed "cancer immunoediting." In this review, we summarize the history of the cancer immunosurveillance controversy and discuss its resolution and evolution into the three Es of cancer immunoediting--elimination, equilibrium, and escape.

MHC class I-deficient metastatic tumor variants immunoselected by T lymphocytes originate from the coordinated downregulation of APM components

Previous reports from our group indicated that the MHC class I phenotype of metastatic lung colonies produced by a mouse fibrosarcoma tumor clone (B9) were, depending on the immune status of the host, MHC class I negative in immunocompetent mice and MHC class I positive in immunodeficient athymic nude/nude mice. Now we report the identification of the molecular alterations responsible for the changes of MHC class I molecules in both situations. Metastatic nodes were analyzed for the mRNA level of H-2 class I and beta2-microglobulin genes, and several gene components of the major histocompatibility complex (MHC) class I antigen-processing machinery (APM). These included the genes coding for the low-molecular-weight proteins LMP2, LMP7, LMP10, the transporter associated with antigen processing (TAP-1, TAP-2), and calnexin, calreticulin, tapasin, PA-28-alpha, PA-28-beta, ERP-59 and ER-60. Analyses with RT-PCR showed that TAP-1, TAP2, LMP-2, LMP7, LMP10, tapasin and calnexin mRNA specific for these genes was absent in metastases produced in immunocompetent mice. In contrast, similar techniques with mRNA preparations obtained from metastatic nodes from immunodeficient mice showed that the mRNA expression level of these genes was highly positive. Interestingly, the MHC class I-positive or negative phenotypes of the metastatic colonies correlated with in vivo immunogenicity. H-2 positive metastasis grew more slowly than the H-2 negative ones when injected intrafootpat in syngeneic immunocompetent animals and were finally rejected. These results provide evidence of the role of T cells in immune surveillance against tumors and identify a mechanism targeted by antitumor T lymphocytes to generate MHC class I-negative tumor escape variants.

MHC class I antigens, immune surveillance, and tumor immune escape

Oncogenic transformation in human and experimental animals is not necessarily followed by the appearance of a tumor mass. The immune system of the host can recognize tumor antigens by the presentation of small antigenic peptides to the receptor of cytotoxic T-lymphocytes (CTLs) and reject the nascent tumor. However, cancer cells can sometimes escape these specific T-cell immune responses in the course of somatic (genetic and phenotypic) clonal evolution. Among the tumor immune escape mechanisms described to date, the alterations in the expression of major histocompatibility complex (MHC) molecules play a crucial step in tumor development due to the role of MHC antigens in antigen presentation to T-lymphocytes and the regulation of natural killer cell (NK) cell function. In this work, we have (1) updated information on the mechanisms that allow CTLs to recognize tumor antigens after antigen processing by transformed cells, (2) described the altered MHC class I phenotypes that are commonly found in human tumors, (3) summarized the molecular mechanisms responsible for MHC class I alteration in human tumors, (4) provided evidence that these altered human leukocyte antigens (HLA) class I phenotypes are detectable as result of a T-cell immunoselection of HLA class I-deficient variants by an immunecompetent host, and (5) presented data indicating the MHC class I phenotype and the immunogenicity of experimental metastatic tumors change drastically when tumors develop in immunodeficient mice.

Cancer immunoediting: from immunosurveillance to tumor escape

The concept that the immune system can recognize and destroy nascent transformed cells was originally embodied in the cancer immunosurveillance hypothesis of Burnet and Thomas. This hypothesis was abandoned shortly afterwards because of the absence of strong experimental evidence supporting the concept. New data, however, clearly show the existence of cancer immunosurveillance and also indicate that it may function as a component of a more general process of cancer immunoediting. This process is responsible for both eliminating tumors and sculpting the immunogenic phenotypes of tumors that eventually form in immunocompetent hosts. In this review, we will summarize the historical and experimental basis of cancer immunoediting and discuss its dual roles in promoting host protection against cancer and facilitating tumor escape from immune destruction.

Lymphocyte-mediated immunosurveillance of epithelial cancers?

The validity and importance of the immunosurveillance of tumors have been debated ever since Burnet and Thomas outlined several key concepts that underpin our understanding of tumor immunity. With the development of gene-targeting technology, it has been shown that interferon gamma (IFN-gamma) and perforin--key effector molecules of both the innate and adaptive immune systems--contribute to the prevention of of tumors in mice. These findings have rejuvenated interest in the immunosurveillance of tumors, and recent data have provided evidence that the immune system influences the development of some spontaneous epithelial malignancies.

Immunoselection by T lymphocytes generates repeated MHC class I-deficient metastatic tumor variants

Alteration of MHC class I molecule expression is a widespread mechanism used by tumor cells to evade T cell responses. It has long been proposed that the origin of these MHC class I-negative or -deficient tumor variants is T cell immune selection. However, there are no experimental or clinical data to substantiate this hypothesis, and this issue is currently the subject of debate. Here we report that an H-2 class I-negative fibrosarcoma tumor clone generated MHC class I-negative spontaneous lung metastases in immunocompetent syngeneic BALB/c mice. Interestingly, the same B9 clone generated MHC class I-positive metastatic nodes, under basal conditions, in athymic nu/nu BALB/c mice. This phenomenon was observed in the metastatic nodules generated after a period of in vivo growth but not in the primary tumors growing locally in the footpad. These findings support the hypothesis that the H-2 phenotype of metastatic nodes is influenced by the T cell repertoire of the host, since in the absence of this T cell pressure (i.e., in nude mice) the metastatic nodes 'recovered' H-2 class I expression. In addition, 2 different phenotypes were found when the metastatic nodules obtained from immunocompetent mice were treated with IFN-gamma. One phenotype, present in 83% of the colonies, was characterized by resistance of the Ld molecule to IFN-gamma induction, due to a deletion involving the Ld gene. The second phenotype (17% of the colonies) was similar to the original B9 clone and was characterized by the response of K, D and L class I genes to IFN-gamma. These data provide evidence that the changes in MHC class I expression during tumor development might not be random but could be predictable.

Immunological enhancement of primary tumor development and its prevention

While it has been known for decades that the growth of tumor transplants can be enhanced immunologically, the potential significance of these previous findings to the development of primary tumors and the mechanisms of tumor enhancement has remained obscure. This review will summarize recent experiments indicating that primary tumor development can be enhanced by active immunization. The evidence suggests that antibodies, B cells and CD4+ T cells can play a critical role in enhancing the development of primary, tumors, whereas endogenous interferon-gamma (IFNgamma) can counteract enhancement. Thus, we envision two possible functions of IFNgamma: (i) preventing B cell and antibody enhancement and (ii) counteracting tumor promotion independent of T and B cells.

CD8+ T cells are crucial for the ability of congenic normal mice to reject highly immunogenic sarcomas induced in nude mice with 3-methylcholanthrene

An attempt was made to identify the selection pressures put upon a growing tumour by CD8+ T cells. To this end tumours induced with 3-methylcholanthrene in T cell-deficient nude mice and in congenic T cell-competent nu/+ mice were transplanted to nu/+ recipients. The rejection rate of the sarcomas from nude mice was almost twice that of the sarcomas from nu/+ mice. Depletion of CD8+ T cells from nu/+ recipients prior to transplantation made them accept nude tumours that were consistently rejected by untreated nu/+ recipients. These findings suggest that a methylcholanthrene sarcoma during its growth in a T cell-competent host adapts to the T cell system through a selective elimination of highly immunogenic tumour cells that are susceptible to CD8+ T cell-mediated lysis.

Immune selection in neoplasia: towards a microevolutionary model of cancer development

The dual properties of genetic instability and clonal expansion allow the development of a tumour to occur in a microevolutionary fashion. A broad range of pressures are exerted upon a tumour during neoplastic development. Such pressures are responsible for the selection of adaptations which provide a growth or survival advantage to the tumour. The nature of such selective pressures is implied in the phenotype of tumours that have undergone selection. We have reviewed a range of immunologically relevant adaptations that are frequently exhibited by common tumours. Many of these have the potential to function as mechanisms of immune response evasion by the tumour. Thus, such adaptations provide evidence for both the existence of immune surveillance, and the concept of immune selection in neoplastic development. This line of reasoning is supported by experimental evidence from murine models of immune involvement in neoplastic development. The process of immune selection has serious implications for the development of clinical immunotherapeutic strategies and our understanding of current in vivo models of tumour immunotherapy.

Differential tumor surveillance by natural killer (NK) and NKT cells

Natural tumor surveillance capabilities of the host were investigated in six different mouse tumor models where endogenous interleukin (IL)-12 does or does not dictate the efficiency of the innate immune response. Gene-targeted and lymphocyte subset-depleted mice were used to establish the relative importance of natural killer (NK) and NK1.1(+) T (NKT) cells in protection from tumor initiation and metastasis. In the models examined, CD3(-) NK cells were responsible for tumor rejection and protection from metastasis in models where control of major histocompatibility complex class I-deficient tumors was independent of IL-12. A protective role for NKT cells was only observed when tumor rejection required endogenous IL-12 activity. In particular, T cell receptor Jalpha281 gene-targeted mice confirmed a critical function for NKT cells in protection from spontaneous tumors initiated by the chemical carcinogen, methylcholanthrene. This is the first description of an antitumor function for NKT cells in the absence of exogenously administered potent stimulators such as IL-12 or alpha-galactosylceramide.

Selection of metastatic tumour phenotypes by host immune systems

Metastasis and the processes underlying this phenomenon make epithelial cancers so malignant. Local control of cancers by surgery is sometimes possible but locoregional and distant recurrence commonly lead to the failure of treatment with ensuing morbidity and mortality. Tumour cells express a range of new antigens during growth and there are opportunities for the host immune system to interact with these antigens. This immune interaction eliminates the tumour or allows selection of phenotypic variants. Cell phenotypes selected by an incomplete immune response resemble the cell type commonly associated with metastases. Thus we propose that the host immune system may be responsible for selection of this phenotype and progression of the disease.

The role of cytotoxic T-lymphocytes in the prevention and immune surveillance of tumors--lessons from normal and immunodeficient mice

The idea of immunological surveillance against cancer has existed for nearly 100 years but as no conclusive evidence has yet been published the importance of the cellular immune defense in the detection and removal of incipient or existing tumors is still a hotly debated subject. However, in order to select a relevant immunotherapeutic strategy in the treatment of cancer, a fundamental understanding of the basic immunologic conditions under which a tumor develops and exists is a prerequisite. Therefore, a murine model was set up that we hoped would enable us to confirm or reject the theory of immunological surveillance. A large panel of methylcholanthrene induced tumors was established in T-cell immunodeficient nude mice and congenic normal mice to study the influence of the immune system on developing tumors. As nude mice developed tumors fastest and with the highest incidence, we concluded that in this model the immune system constituted a 'tumor-suppressive factor' delaying and sometimes abrogating tumor growth, i.e. performing immune surveillance. Immunogenicity of the tumors was assessed by transplantation back to normal histocompatible mice. Tumors originating from the immunodeficient nude mice turned out to be far more immunogenic than tumors from normal mice, resulting in a high rejection rate. CD8+ cytotoxic T cells were found to be indispensable for this rejection, leading to the conclusion that the cytotoxic T cells perform immune selection in normal mice, eliminating immunogenic tumor cell variants in the incipient tumor. In this review, we discuss the difficulties facing immunotherapy when conclusions are drawn from the presented observations and hypotheses.

Susceptibility to the biological effects of polyaromatic hydrocarbons is influenced by genes of the major histocompatibility complex

Polyaromatic hydrocarbons are ubiquitous environmental chemicals that are important mutagens and carcinogens. The purpose of this study was to determine whether genes within the major histocompatibility complex (MHC) influence their biological activities. Cell-mediated immunity to dimethylbenz(a)anthracene (DMBA) was investigated in congenic strains of mice. On three different backgrounds, H-2(k) and H-2(a) haplotype mice developed significantly greater contact-hypersensitivity responses to DMBA than H-2(b), H-2(d), and H-2(s) mice. In B10.A(R1) mice, which are Kk and Id, a vigorous contact-hypersensitivity response was present, indicating that the response was governed by class I, rather than class II, MHC genes. C3H/HeN (H-2(k)) and C3H.SW (H-2(s)) strains were also compared for the development of skin tumors and the persistence of DMBA-DNA adducts. When subjected to a DMBA initiation, phorbol 12-tetradecanoate 13-acetate (TPA)-promotion skin-tumorigenesis protocol, C3H/HeN mice, (which develop cell-mediated immunity to DMBA) were found to have significantly fewer tumors than C3H.SW mice (a strain that failed to develop a cell-mediated immune response to DMBA). DMBA-DNA adducts were removed more rapidly in C3H/HeN than in C3H.SW mice. The results indicate that genes within the MHC play an important role in several of the biological activities of carcinogenic polyaromatic hydrocarbons. The observations are consistent with the hypothesis that cell-mediated immunity to chemical carcinogens serves to protect individuals by removing mutant cells before they can evolve into clinically apparent neoplasms.

High MHC class I expression correlates with slow growth in UV-induced skin carcinomas in hairless mice

An experiment was set up to investigate the relationship, if any, between cell surface MHC class I expression and the growth rate for skin tumors induced by two different UV radiation regimens in hairless mice. Two groups of 20 hairless mice were each irradiated with either a UVA radiation source (2 SED per session) or broad-spectrum UV radiation (UVB) (8.1 SED per session) 5 days a week during the entire experiment. In the UVA group, 17 out of 20 animals developed tumors, and 10 of these grew to a diameter of > or = 5 mm. In the UVB group, 19 out of 20 animals developed tumors, and 15 of these grew to a diameter of > or = 5 mm. The tumor induction time, i.e. the time from the start of UV treatment to tumor appearance, was found to be significantly longer (p<0.01) in the UVA than in the UVB group. This is in accordance with previous findings. Of the 25 tumors growing to a diameter of > or = 5 mm, 11 were established as cultured cell lines (4 UVA and 7 UVB tumors). These uncloned cell lines were analyzed for surface expression of major histocompatibility complex class I by FACS analysis. There was a clear correlation between high MHC class I expression and slow growth of the individual tumors (p<0.05). This suggests a role for the MHC class I governed, i.e. cytotoxic T-cell-mediated, reactions in deciding the fate of UV-induced skin cancers. No correlation was found between MHC class I expression and tumor induction time.

Methylcholanthrene-induced sarcomas in nude mice have short induction times and relatively low levels of surface MHC class I expression

In order to study the role of the T-cell-mediated immune defense in tumor development, a total of 93 sarcomas were induced using different doses (8 micrograms (0.1%), 40 micrograms (0.5%) and 400 micrograms (5%)) of 3-methylcholanthrene in athymic nude Balb/c mice and phenotypically normal immunocompetent Balb/c mice. A shorter tumor induction time and a higher tumor incidence after treatment with low doses of methylcholanthrene were seen in nude mice than in immunocompetent mice, indicating that they have a lower resistance to the carcinogen. Contrary to expectations we found that the MHC class I expression of tumors from nude mice was lower than that of tumors from normal mice. Higher surface expression of MHC class I was demonstrated on high dose tumors from normal mice than on low dose tumors from normal mice. The cellular composition of the individual tumors raised in nude mice was more heterogeneous with respect to MHC class I expression. Since the mice differ genetically only with respect to the nu gene, these results indicate that a lack of T-cell-mediated defense mechanisms may confer upon the bearer a lower resistance to 3-methylcholanthrene and a different MHC profile of the ensuing tumor.